Thermal conversion of oil shale into recoverable hydrocarbons



Sept. 23, 1969 M. L. SLUSSER ETAL 3,468,376

THERMAL CONVERSION OF OIL SHALE INTO RECOVERABLE HYDROCARBONS Filed Feb. 10. 1967 HEATING 48 FLUID ABRASIVE 26 :7 PARTICLES I I I. FLOWCHANNEL I. I k 63 FLOW CHANNEL INVENTORS MARION L. SLUSSER WILLIAM E. BRAMHALL ATTORNEY United States Patent US. Cl. 166-272 5 Claims ABSTRACT OF THE DISCLOSURE The production of hydrocarbons by pyrolysis of oil shale with controlled removal of the resulting layer of spent oil shale residue. More particularly, a procedure is described for the in situ thermal conversion of oil shale wherein fluidized abrasive particles are employed to foster improved hydrocarbon production, in amount and kind, by a controlled partial removal of the layer of spent oil shale which results from application of flowing fluids to heat exposed surfaces of the oil shale to release hydrocarbons.

BACKGROUND OF THE INVENTION This invention relates to the production of hydrocarbons from oil shale, and more particularly, to the in situ pyrolysis of oil shale for this result.

Vast sources of hydrocarbons are the massive oil shale formations dispersed in several locations in the world. Oil shale contains a solid carbonaceous material known as kerogen. Kerogen is converted to fluid hydrocarbons by heating the oil shale to temperatures above about 500-700 F. After the kerogen is converted to hydrocarbons, there remains a spent inorganic residue of the depleted oil shale.

In the thermal conversion, or pyrolysis, of the oil shale, the rate of hydrocarbon production depends upon the heat and mass transfer properties of the oil shale. In one in situ process, circulating heated fluids may be used for pyrolysis of the oil shale. In situ combustion procedures also may be used for pyrolysis of the oil shale. In each instance, the required heating of the oil shale is provided by a hot fluid contacting the oil shale. However heating is obtained, the properties of the oil shale control the ultimate production of hydrocarbons.

There are two mechanisms involved in the transport of heat through the oil shale. Heat is transferred through the solid mass of oil shale by conduction. The heat is also transferred by convection through the solid mass of oil shale. The transfer of heat by conduction is a relatively slow process. The average thermal conductivity and average thermal diflusivity of oil shale are about those of a firebrick. The matrix of solid oil shale has an extremely low permeability much like unglazed porcelain. As a result, the convective transfer of heat is limited to heating by fluid flows obtained in open channels which traverse the oil shale. These flow-channels may be natural and artificially induced fractures.

The problems due to the restrictive heat and mass transfer properties of the oil shale may be illustrated as follows. Consider a recovery process in oil shale employing in situ combustion to generate the heat for the thermal conversion of kerogen into hydrocarbons. Fluid flow, in a channel, provides the oxidant, usually air, to maintain the in situ combustion of the combustible matter present in the oil shale. The fluid flow also serves as the driving media for production of the released hydrocarbons into the channel. On heating, a layer of pyrolyzed oil shale builds adjacent the channel. This layer is an inorganic, mineral-matrix which contains varying degrees of carbon.

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The layer is an ever-expanding barrier to heat flow from the heating fluid in the channel. The layer also is a barrier to the flow of the produced hydrocarbons from the oil shale being thermally converted beyond this layer. When the resistance in this layer to heat transfer, and to fluid flow, exceeds the available driving forces carrying out the thermal conversion of the oil shale, the process of producing recoverable hydrocarbons will cease.

SUMMARY OF THE INVENTION The present invention is a method having cooperating steps wherein a heating fluid is circulated in a flow-channel through oil shale for heating its exposed surfaces to pyrolyze kerogen into fluid products, and to leave as a residue, a self-supporting layer of spent oil shale. Abrasive particles are added to the heating fluid to erode the layer to a thickness wherein the production of hydrocarbons from the oil shale being pyrolyzed adjacent the channel is obtained at optimum high levels for a given set of pyrolyzing conditions. Thereafter, the heating fluid is circulated with the addition thereto of abrasive particles needed to erode the layer of oil shale subjected to pyrolysis to maintain this optimum hydrocarbon production. The hydrocarbons are recovered in the fluids produced from pyrolysis of the oil shale adjacent the flow channel. The abrasive particles may be continuously added at a uniform rate to the heating fluid. Alternatively, these particles may be added at nonuniform addition rates with variation in rates adjusted to maintain hydrocarbon production at substantially the optimum high level. Further, the amount of added abrasive particles, and the pyrolysis conditions may be adjusted to maintain the production of certain hydrocarbons at optimum high levels.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a vertical section of a subterranean formation of oil shale provided with suitable apparatus for carrying out the steps of the present method;

FIGURE 2 is a graphic representation in vertical section of an oil shale sample in which conventional in situ combustion and retorting have occurred; and

FIGURE 3 is a graphic representation, similar to that of FIGURE 2, but of an oil shale sample which was subjected to pyrolysis by the method of this invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS Referring to FIGURE 1 of the drawings, there are shown several earthen formations; an oil shale 11 residing below the earths surface 12, a surface layer 13, and an overburden 14. The oil shale 11 rests upon a supporting formation 16. For purposes of this description, the formations 13, 14, and 16 may be considered to be impervious and without carbonaceous materials. The oil shale 11 is penetrated by spaced-apart fluid entry means and fluid recovery means which may be an injection well 17 and a production well 18. These wells are suitably constructed to provide fluid communication between the earths surface 12 and the oil shale 11. For example, the injection well 17 has a casing 19 extending downwardly into a wellbore 21 penetrating the oil shale 11. The casing 19 is sealed by a cement sheath 22 to a portion of the surrounding oil shale 11 and to formations 13 and 14. If desired, the casing 19 may extend the full depth of the wellbore 21 with suitable fluid entries, such as perforations, to the adjoining oil shale 11. Where the oil shale 11 is self-supporting about the wellbore 21, an open-completion arrangement for the injection well 17 can be employed. The injection well 17 carries a wellhead apparatus 23 through which passes a conduit 24. The conduit 24 has inlets 26 and 27 through which fluids can be introduced selectively into the conduit 24.

The production well 18 is arranged similarly to the injection well 17. The well 18 has a casing 31 extending the depth of a wellbore 32 penetrating the oil shale 11. Perforations 33 in the lower extremity of the casing 31 serve as fluid entries to the adjacent oil shale 11. The casing 31 is sealed by a cement sheath 34 at its upper extremity to a portion of the adjoining oil shale 11, and to formations 13 and 14. The well 18 carries a wellhead apparatus 36 through which passes a conduit 37. Preferably, the conduit 37 extends downwardly into the Wellbore 32 to adjacent the lower extremity of the well 18 so that fluids are readily produced from the lower depths of the oil shale 11.

The oil shale 11 usually contains at least one natural flow-channel through which a heating fluid can be circulated. For example, the oil shale 11 may contain, as a flow-channel, natural fractures 41 and 42 extending in fluid communication between the injection well 17 and the production well 18. If desired, artificial fractures 43 and 44 are extended between these wells as flow-channels. The artificial fractures 43 and 44 are provided by any means, such as by hydraulic fracturing. Thus, a channel for fluid flow is provided by the fractures 41, 42, 43, and 44 through the oil shale 11 between the wells 17 and 18.

A heating fluid is circulated from the injection well 17 through the fractures 41 through 44 to the production well 18. By this means, the oil shale 11 adjacent these fractures is heated to temperatures sufficient for converting, by pyrolysis, the kerogen into fluid hydrocarbons. These hydrocarbons are recovered with the fluid flows through the conduit 37 of the production well 18.

The heating fluid must heat the oil shale 11 to the necessary pyrolysis temperatures for producing hydrocarbons from kerogen. For example, the heating fluid may be an inert fluid which is heated to suitable temperatures either at the earths surface, or within the wellbore 21. This fluid then is passed into the fractures 41 through 44 for retorting the oil shale 11.

However, it is preferred to carry out in situ combustion within the oil shale 11 to provide the fluid for heating the exposed surfaces of the oil shale 11 to the necessary temperatures for producing hydrocarbons. For this purpose, the heating fluid may be an oxygen-containing gas introduced through the inlet 26 into the conduit 24 to pass through the oil shale 11 via the fractures 41 through 44, and ultimately to flow into the production well 18. With the passage of such fluid, the oil shale 11 adjacent the wellbore 21 is ignited by suitable means and the desired in situ combustion is obtained. A resulting combustion zone or front expands along the fractures 41 through 44 in the oil shale 11 in a direction of movement toward the production well 18. As a result, the oil shale 11 reaches suitable pyrolysis temperatures, and the released hydrocarbons which enter the fractures 41 through 44 ultimately flow into the production well 18. All fluids entering the production well 18 are removed through the conduit 37 and then passed to suitable means wherein the hydrocarbons are recovered.

As the heating fluid passes through the fractures 41 through 44, a layer of ever-increasing thickness of pyrolyzed oil shale is built as hydrocarbons are released from the oil shale 11.

When the layer of spent oil shale begins to build in thickness, abrasive particles are added to the heating fluid entering the injection well 17. These particles erode the layer of pyrolyzed oil shale lining the fractures 41 through 44. More particularly, the abrasive particles are introduced at a metered rate, from a pump 38 through the inlet 27 into the conduit 24 where they mix with the heating fluid entering through the inlet 26. The abrasive particles then pass into the oil shale 11 from the injection Well 17 through the conduit 24. By this means, the amounts of the abrasive particles admixed with the heating fluid are regulated.

The abrasive particles may be of any form and composition that provide for eroding of the layer of spent oil shale at high temperatures. For example, these particles can be abrasive materials such as Ottawa sand, aluminum oxide, or silicon carbide, separately or in admixture. Other abrasive materials can be used, if desired. The abrasive particles should be of suificient fineness as to be carried in the fluid flows used for pyrolysis of the oil shale 11. For example, abrasive particles having a size in the range of 30-60 mesh, as defined by the Tyler standard screen scale, can be used. Other particle sizes can be used if desired.

The entire layer of residue produced by thermal conversion could be removed continuously to provide a fresh surface of oil shale exposed to heating fluids. However, in the present invention, only a part of this layer of retorted oil shale is removed adjacent the flow-channel wherein heating fluid is circulated through the oil shale. The retained layer produces many beneficial results besides supplying fuel for combustion. One result is the thermal upgrading of the pyrolysis products from kerogen into more desirable hydrocarbons. Another result is that the exposure of the released hydrocarbons directly to a flow of heated fluid which contains free oxygen is reduced. This provides a considerable reduction in loss by combustion of these hydrocarbons. These beneficial results are obtained in this invention while the undesired effects of this layer are substantially reduced.

The addition of the abrasive particles is controlled so that the thickness of the layer of pyrolyzed oil shale is reduced but not totally eliminated. The thickness of this layer is reduced to a certain dimension where the production of hydrocarbons from the unpyrolyzed oil shale 11 adjacent the fractures 41 through 44 is obtained at optimum high levels for a given set of heating conditions.

This certain thickness in the layer of pyrolyzed oil shale is of advantage by preventing the released hydrocarbons from being subjected to combustion upon entering directly into the hot heating gas, containing free oxygen, flowing within the fractures 41 through 44. A greater thickness of the layer of pyrolyzed oil shale provides a substrate in which the released hydrocarbons are converted by a thermal cracking operation to noncondensable gaseous products. The hydrocarbons initially produced by pyrolysis of kerogen are of high molecular weights, being mainly of a resinous origin. However, passage of these hydrocarbons through the retained thickness in the layer of the pyrolyzed oil shale cracks them into more usable liquid hydrocarbons.

For a given set of pyrolyzing conditions, there is a certain thickness in the layer of pyrolyzed oil shale which products an optimum production of hydrocarbons. The amounts of abrasive particles to erode the layer to this thickness are adjusted until this condition is reached. The amounts and quality of hydrocarbons flowing into the well 18 may be monitored for this purpose.

Preferably, the abrasive particles are added continuously to the heating fluid and at a uniform rate. However, the abrasive particles may be added at a nonuniform rate to the heating fluid, but under such conditions that the variation in the ratio of addition is adjusted to maintain, by averaging, the total production of hydrocarbons at substantially the optimum high levels.

Where a particular range of molecular weights i desired in the hydrocarbons to be produced, the amounts of abrasive materials added to the heating fluid, and the conditions regulating pyrolysis of the oil shale, are adjusted until production is obtained of these certain hydrocarbons and at optimum high levels.

At the present time, the exact mathematical interrelationship of the various pyrolysis conditions cannot be set forth for the preceding steps. However, it is to be understood that these conditions may be varied one at a time until the optimum high levels in the production of the hydrocarbons are obtained. For example, there is a satisfactory balance obtainable between the amount of abrasive particles added to a heating fluid and the amount of free oxygen it contains, to produce a certain hydrocarbon production. Variation in each of the other conditions of pyrolysis provides corresponding changes in the production of hydrocarbon at the production well 18.

Various means can be used for monitoring pyrolysis conditions in controlling the addition of the abrasive particles passed through the fractures 41 through 44. For example, as is shown in the drawings, the conduit 37 in the well 18 is connected to a separator 39 wherein the abrasive particles are removed as a stream 46. The hydrocarhon-containing fluids flow from the separator 39 through a conduit 48. The fluids, in the conduit 48, are sent to a suitable recovery system (not shown) for separating the hydrocarbons from undesired coproduced fluids. A monitor 47 is connected to the conduit 48 and measures the hydrocarbon contents, oxygen, argon, carbon dioxide, etc. of the fluid stream from the well 18. The monitor 47 may be a chromatograph or mass spectrometer. The monitor 47 also provides, by a connection indicated by chain line 49 with the pump 38, for adjusting its speed so that variation in the amount of hydrocarbons contained in the conduit 48 regulate the amount of abrasive particles intermixed with the heating fluid entering the injection well 17. Various other arrangements of the monitor 47, the connection 49, and the pump 38 can be employed for carrying out the purposes of this invention, if desired.

The circulation of the heating fluid and addition of abrasive particles from the injection well 17 through the fractures 41 through 44 to the production well 18 are continued to maintain the optimum hydrocarbon production during pyrolysis of the oil shale 11. The hydrocarbons in the fluids produced into the production well 18 are conveyed in the conduit 48. Any suitable procedure may be employed for recovering the hydrocarbons from such produced fluids.

Experimental results have substantiated the efliciency of the present method in the pyrolysis of oil shale. The in situ combustion pyrolysis of oil shale to produce hydrocarbons was simulated in the laboratory with cylindrical cores of oil shale having a recoverable hydrocarbon content of about 35 gallons per ton. The results of certain comparative tests are shown by graphic illustration in FIGURES 2 and 3.

In FIGURE 2, a cross section through a cylindrical core of the oil shale is displayed. A small flow-channel 51 was formed axially through the oil shale 52. The oil shale 52, enclosed within a cylindrical retort, received a flow of heating gas through the flow-channel 51, as indicated by the arrows. The heating gas was air preheated to about 1600" F. The gases from the outlet of the channel 51 were tested to determine their hydrocarbon content. Air, at the indicated temperature, was passed through the channel 51 for a. length of time to provide a sufliciently thick layer of pyrolyzed oil shale adjacent the channel 51 that reduced drastically the amount of hydrocarbons produced from the oil shale 52.

Examination of the oil shale 52 indicated that this layer was actually formed of two basic parts. Immediately adjacent the passage 51 was a layer 53 of spent oil shale substantially burned free of carbonaceous material. The layer 53 was surrounded by a layer 54 of spent oil shale which retained some carbonaceous material. The layers 53 and 54 were of sufficient thickness that the production of hydrocarbons ceased. The resistance to heat transfer, and to fluid flow produced by the layers 53 and 54 was great. Under these conditions, the available driving forces of the in situ combustion carried out in the passage 51 were insuflicient to produce paying quantities of hydrocarbons. The conclusion associated with conventional in situ pyrolysis was correct; there is eventuall built up a layer of spent oil shale having such thickness that for practical purposes the production of substantial quantities of hydrocarbons terminates relatively soon after the initiation of pyrolysis along a surface exposed to heating fluids.

In FIGURE 3, a cylindrical core of oil shale 63, similar to the one displayed in FIGURE 2, is shown as being processed in accordance with the present invention. The core of oil shale 63 was placed into a retort and provided with a flow-channel 61 receiving a flow of air at 1600 F. A mass spectrometer was provided for testing the hydrocarbon content of fluids flowing from the outlet end of the passage 61. The heated air flowed in the direction of the arrows through the passage 61. The heated air was passed at a velocity of about 5-25 ft./sec. through the channel 61 in the oil shale 63. Small amounts of silicon carbide particles, between 30 and 60 mesh, were added periodically to the air. Between the additions of the abrasive particles, a layer 64, like layers 53 and 54, of spent oil shale was built up adjacent the passage 61. The production of hydrocarbons from the outlet of passage 61 quickly ceased when the flow of abrasive particles was terminated for extended intervals. For a period of time thereafter, the oxygen and carbon dioxide content in the gases issuing from the passage 61 continued to rise. This indicates the undesired effects of the increasing thickness of the layer 64 on the production of hydrocarbons.

The introduction of abrasive particles into the heating fluid, to remove the entire layer 64 of pyrolyzed oil shale would not be economically feasible. But, it is also not economically feasible to allow the layer of pyrolyzed oil shale to build to such thickness on the oil shale 11 that the conversion of kerogen into recoverable hydrocarbons terminates. By this invention, abrasive particles are added to the heating fluid flowing through the flow channel in the oil shale 11 in just an amount to reduce the thickness of the layer of pyrolyzed oil shale until the beneficial effects of the layer are obtained but its undesired effects are avoided. As a result, the remaining thickness of the layer of the pyrolyzed oil shale provides for the production of hydrocarbons in the unpyrolyzed oil shale, adjacent flowchannels, to be maintained at optimum high levels for a given set of pyrolyzing conditions.

The foregoing description obviously has disclosed a method with combined steps for avoiding the problems heretofore found in prior art methods for pyrolyzing oil shale into recoverable hydrocarbons. It is intended that this description be illustrative but not limitative of this invention, which invention is defined by the appended claims. It will be appreciated that various changes and alterations may be made to the steps of this method with out departing from the spirit of the invention, and such arrangements are intended to reside within the scope of these claims.

What is claimed is:

1. A method for the thermal conversion of oil shale into recoverable hydrocarbons comprising the steps of:

(a) providing a continuous flow-channel through said oil shale between spaced-apart fluid entry means and fluid recovery means;

(b) circulating through said flow-channel in said oil shale between said means a fluid for heating exposed surfaces of said oil shale along said channel to temperatures sufficient to pyrolyze kerogenic material" into gases and liquid products, and leaving .as a resi" due a layer of self-supporting mineral matrix of pyrolyzed oil shale;

(c) adding abrasive particles to said heating fluid to erode the layer of pyrolyzed oil shale adjacent said flow-channel, and said addition occurring in amounts to reduce the thickness of said layer of pyrolyzed oil shale to a suflicient dimension wherein the production of hydrocarbons in the unpyrolyzed oil shale adjacent said channel is obtained at optimum high levels for said given set of pyrolyzing conditions; and

(d) continuing to circulate the heating fluid for pyrolyzing said oil shale with the addition of the abrasive particles needed to erode the layer of oil shale subjected to pyrolysis to maintain the optimum hydrocarbon production, and recovering hydrocarbons in the fluids produced from pyrolysis of said oil shale adjacent said channel.

2. The method of claim 1 wherein the abrasive particles are continuously added at a uniform rate to said heating fluid.

3. The method of claim 1 wherein the abrasive particles are added at a nonuniform rate to said heating fluid with the variation in the rate of addition adjusted to maintain the production of hydrocarbons at substantially the optimum high level.

4. The method of claim 1 wherein the amount of abrasive particles added to said heating fluid, and the conditions regulating pyrolysis of said oil shale, are adjusted to maintain the production of certain hydrocarbons at optimum high levels.

5. The method of claim 1 wherein natural fractures in said oil shale provide a continuous subterranean flowchannel between said spaced-apart fluid entry and fluid recovery means through which pyrolysis to provide for the production of hydrocarbons is obtained.

References Cited UNITED STATES PATENTS 2,796,129 6/1957 Brandon 166-42 X 2,825,408 3/1958 Watson 166-11 2,974,937 3/1961 Kiel 166-11 X 3,004,594 10/1961 Crawford 166-11 3,010,512 11/1961 Hurley et al. 166-11 3,149,670 9/1964 Grant 166-11 3,233,668 2/1966 Hamilton et al. 166-11 X 3,241,611 3/1966 Dougan 166-11 X 3,284,281 11/1966 Thomas 166-11 X 3,362,471 l/1968 Slusser et al. 166-39 X 3,400,762 9/1968 Peacock et al. 166-11 STEPHEN J. NOVOSAD, Primary Examiner US. Cl. X.R. 166-251, 259 

